Add feature to set session keys from existing keystore keys

[PierreBesson]

The session-key-grabber, already contains logic to get sessions public key as string. We should write the automation to submit the setKeys extrinsic with the Validator/Collator account.

This is an alternative to the rotate_session_keys APIs that generates new keystore keys on the nodes and allows to use statically defined keys (eg. from secret or Vault).